The present invention relates to pharmaceutical compositions and methods of drug delivery. The invention especially relates to methods and compositions for controlled release of easily denatured drugs utilizing microcapsules.
Microencapsulation technologies have advanced significantly during the last few decades and the current technologies are at such a level that drugs can be delivered at predetermined rates for days and years depending on applications. These advances, however, usually apply only to low molecular weight drugs. Microencapsulation of high molecular weight drugs, such as peptides and proteins, is still complicated due to the intricate nature of their physical and chemical properties. Of these, protein-based pharmaceuticals have become especially important since mass production of such drugs has been enabled by recombinant DNA technology. Furthermore, completion of the human genome project is expected to bring about an improved understanding of the therapeutic roles of specific proteins, which should lead to numerous new protein drugs.
Since the early promise of sustained delivery of proteinaceous drugs [1], studies on protein microencapsulation have increased exponentially. Some of the early efforts succeeded in bringing the first microparticle product for peptide delivery (LUPRON DEPOT) onto the market [2, 3]. Not long after the commercial success of this product, however, it was recognized that the susceptibility of most proteins to environmental stresses would pose serious barriers to development of microparticle systems for proteins. Despite all the obstacles, protein microencapsulation is still an attractive approach, especially when such pharmaceuticals cannot be delivered via oral routes. Also, the need for infusions or frequent drug injections calls for the development of long-term delivery systems.
Current technologies commonly used in the preparation of microparticles for controlled drug delivery are summarized below, each of which has its own advantages and limitations.
Emulsionxe2x80x94solvent evaporation or extraction
Coacervation (Simple and complex coacervation)
Hot melt microencapsulation (congealing)
Interfacial cross-linking and interfacial polymerization
Spray drying
Supercritical fluid
The emulsion (solvent evaporation or extraction) methods utilize volatile organic solvents for dissolving water-insoluble polymers, such as poly (lactic acid-co-glycolic acid) (PLGA). Microdrops are produced when a mixture of a drug and a polymer solution is emulsified in a continuous phase, which is usually water. The microdrops become microparticles after solvent removal. A double emulsion process is commonly used for encapsulation of water-soluble drugs such as protein or peptide. Both solid/oil/water (s/o/w) and water/oil/water (w/o/w) systems are used depending on the type of protein drug. In the solvent evaporation method, the organic solvent is removed by evaporation at a raised temperature and/or under vacuum. See, for example, U.S. Pat. No. 3,523,906 (issued to Vrancken, et al. [41]). In the solvent extraction method, the organic solvent is extracted into a large volume of continuous phase, thereby turning the emulsion drops into solid polymer microspheres. See, for example, U.S. Pat. No. 4,389,330 (issued to Tice, et al. [5]). The organic solvents conventionally employed in this method are typically chlorinated hydrocarbons, such as methylene chloride. In both methods, the use of chlorinated solvents often becomes a substantial drawback for environmental and human safety reasons. FDA guidelines for the residual solvents limits the permitted daily exposure to 6 mg per day for methylene chloride. In addition, the loading capacity of microspheres prepared by this method is in general very low. Furthermore, the way emulsions are created increases not only the total interfacial area the bioactive substances are subjected to, but also extensive shear and cavitation stress, which may be destructive to bioactive substances [6].
To minimize such loss of activity of the bioactive substances, it is proposed to make microspheres at very low temperatures. See, e.g., U.S. Pat. No. 5,019,400 (issued to Gombotz, et al. [7]). Biodegradable polymer is dissolved in an organic solvent, such as methylene chloride, together with a protein powder and then atomized over a bed of frozen ethanol overlaid with liquid nitrogen. The microdrops freeze upon contacting the liquid nitrogen and then sink onto the frozen ethanol layer. As the ethanol layer thaws, the frozen microspheres sink into the ethanol. The methylene chloride solvent in the microspheres then thaws and is slowly extracted into ethanol, resulting in hardened microspheres containing proteins and polymer matrix. Clearly, this method can be costly and laborious, especially for scaled-up production.
The coacervation method is based on salting out (or phase separation) of polymers from a homogeneous solution into small drops of a polymer-rich phase upon addition of extra substances. For example, when an aqueous polymer solution (e.g., gelatin or carboxymethylcellulose) is partially dehydrated (or desolvated) by adding a strongly hydrophilic substance (e.g., sodium sulfate) or a water-miscible non-solvent (e.g., ethanol, acetone, dioxane, isopropanol, or propanol), the water-soluble polymer is concentrated to form the polymer-rich phase. This is known as simple coacervation. If water-insoluble drug particles are present as a suspension or as an emulsion, the polymer-rich phase is formed on the drug particle surface to afford a capsule. In complex coacervation, the polymer-rich complex (coacervate) phase is induced by interaction between two dispersed hydrophilic polymers (colloids) of opposite electric charges. Since electrostatic interactions are involved, it is important to control the pH of the medium in order to control the charges of the polymers.
In hot melt microencapsulation (also called congealing), a drug is mixed with a polymer, which is melted at high temperatures. The mixture is then suspended in a non-miscible solvent with continuous stirring at a temperature several degrees above the melting point of the polymer. After the emulsion is stabilized, the system is cooled until the polymer particles are solidified. This method requires the drug to be stable at the polymer melting temperature.
Interfacial cross-linking and interfacial polymerization employ two reactive phases that can form a solid boundary at their interface, which becomes the surface of microparticles. For interfacial cross-linking, the polymer must possess functional groups that can be cross-linked by ions or multi-functional molecules contained in a continuous phase. Interfacial polymerization requires two reactive monomers dissolved in immiscible solvents that can be polymerized at the interfaces. Capsules are collected after quenching the polymerization reaction with a third phase.
Spray-drying involves spraying a mixture of a drug and a polymer and evaporating the solvent in a drying chamber to solidify the atomized drops. This seemingly simple process has not been widely used in the pharmaceutical industry due at least in part to difficulties in the scale-up process. The parameters optimized in the laboratory scale spray dryer do not usually work for the industrial scale spray dryer. Moreover, the temperature of inlet gas that is used for drying the microdrops can reach 90-150xc2x0 C. [9, 10], which may not be tolerable for encapsulation of heat-sensitive biomaterials.
Ultrasonic atomizers have reportedly been used for microparticle formation in connection with the spray-drying or spray-congealing techniques discussed hereinabove. In one example, the ultrasound energy was used to break up drug and carrier mixture into microparticles [11]. A lipid excipient and a drug were mixed at a temperature higher than the melting point of the excipient. The resulting fluid was poured onto the oscillating surface, and the liquid was atomized into small drops upon hitting the surface. The microdrops were collected in a cold chamber in which the liquid drops were solidified (spray-congealing). In another example, a water/oil (w/o) emulsion consisting of an aqueous bovine serum albumin (BSA) solution and PLGA solution in methylene chloride was atomized using an ultrasonic atomizer adapted to a conventional spray dryer [12]. Thus formed microspheres were collected directly after the solvent evaporation (spray-drying). For example, protein loaded microspheres were produced using an ultrasonic atomizer (U.S. Pat. No. 5,389,379, issued to Dirix et al. [13]). They collected the particles in a non-solvent bath to remove the organic solvent. The hardened drops were subsequently transferred into the second non-solvent to harden the microspheres.
Supercritical fluids have recently been utilized for their unique characteristics: high compressibility and liquid-like density. Microparticles have been prepared by either rapid expansion of supercritical solutions (RESS) or supercritical antisolvent crystallization (SAS) [8]. RESS exploits the liquid-like solvent power of the supercritical fluids whereas SAS uses supercritical fluid as an antisolvent. Carbon dioxide is most commonly used because it is environmentally benign, relatively non-toxic, non-inflammable, and inexpensive, and the condition for the critical fluid is easily attainable. RESS is limited by the requirement that all solutes should be soluble in the supercritical fluid. For this reason, RESS may not be used to make protein loaded polymeric microcapsules. By contrast, the SAS method utilizes the supercritical fluid as an anti-solvent that causes precipitation of the solids. Therefore, the SAS method is suitable for solids that are difficult to solubilize in supercritical fluids, such as peptides and proteins. However, the supercritical fluid approach is in its infancy and it is as yet hard to anticipate mass-production of microparticles using this method.
In discussing different approaches of microencapsulation, it is useful to understand the terms commonly used in the microencapsulation field. Microparticles can be categorized as xe2x80x9cmicrospheresxe2x80x9d and xe2x80x9cmicrocapsulesxe2x80x9d. The term xe2x80x9cmicrospheresxe2x80x9d usually refers to monolithic type formulations in which the drug molecules are dispersed throughout the polymeric matrix [14]. On the other hand, the term xe2x80x9cmicrocapsulesxe2x80x9d refers to reservoir devices in which a drug-containing core is surrounded by a continuous polymeric layer, shell, or membrane. Depending on the geometry of the core, microcapsules can be multinuclear microcapsules, where multiple drug cores are embedded throughout the polymer matrix, or mononuclear microcapsules, for which a single drug core is surrounded by the polymer membrane [15].
One of the disadvantages of microspheres or multinuclear microcapsules is that degradation products of the polymer can easily build up to generate acidic microenvironments within the microparticles, which can be undesirable for acid-labile drugs [16]. Furthermore, the presence of abundant polymer in proximity to a large amount of drug can cause unfavorable interaction between two substances. It can be a significant problem when it comes to protein drugs, which are highly susceptible to denaturation due to hydrophobic interactions with the polymer [17].
In this regard, mononuclear microcapsules can provide a number of advantages. First of all, microcapsules provide much more drug reservoir space than microspheres. The reservoir space can accommodate protective excipients as well as drugs. Also, drugs located in the single core are not in extensive contact with the polymer, but only those on the surface are exposed to the degrading polymer. The degradation products of the polymer would not build up within the microcapsules because they are more likely to diffuse out to the release medium rather than to the core structured by the constituent excipients. In addition, the release profile can be further modified by building heterogeneous layers of membranes.
Most previous methods described to date afford either microspheres or multinuclear microcapsules. Most methods produce a mixture of a drug and a polymer in forms of homogeneous solution, suspension, or emulsion, prior to breaking it up into microdrops. It is the mechanics leading to the fragmentation of the mixture that characterizes each method. That is, emulsification in an emulsion method, air atomization in spray drying, and a change in solubility of the polymer induced by the extra substance in the coacervation method.
Mononuclear microcapsules can be produced by the interfacial cross-linking and interfacial polymerization methods. However, the complexity of the procedures and handling of the unreacted monomers can be issues that make them impractical. Gustavsson et al. [18] describe a method to coat starch microparticles with a polymer shell for control of the drug release profile. The polymer coating is obtained by suspending the core microparticles in an air-suspension coater providing a polymer solution [19]. However, the time and the labor required to finish the multi-step procedures may become obstacles when modifying the production scale.
Accordingly, it is an object of the present invention to identify a novel microencapsulation technique that affords such desirable features as the following: (a) entrapment of bioactive substances in microparticles in such a way that their bioactivity is maintained; (b) avoidance of exposure of the encapsulated bioactive substances to undesirable conditions, including a high water-oil interfacial area, contact with the toxic organic solvents or reactive radicals, mechanical stresses, and both extremes of temperature; (c) avoidance of extensive contacts between the bioactive substance and the hydrophobic polymer matrix and/or accumulation of the degradation products of the polymer, which are likely to compromise the stability of the encapsulated drugs; and (d) microencapsulation accomplished in a single-step, thereby permitting easy scale-up of production.
The present invention achieves the objectives identified above by employing atomization technology, particularly ultrasonic atomization, in conjunction with xe2x80x98solvent exchangexe2x80x99 to produce microencapsulated particles. A microencapsulation method based on solvent exchange, which employs ink-jet technology, has been described previously, see, e.g., U.S. Patent Publication 2002/0160109 A1. Surprisingly, it has now been observed that the solvent exchange process can also be achieved by simultaneously producing a large number of drops of aqueous and polymer solutions, and spatially concentrating them to facilitate collision among the drops.
Accordingly, in one aspect of the invention a method for preparing a microencapsulated composition is contemplated. Such a method comprises: (1) providing an aqueous solution containing a composition to be encapsulated dissolved therein; (2) providing a polymer solution containing a water-insoluble polymer dissolved in a hydrophilic solvent; (3) generating a plurality of first microdroplets from the aqueous solution; (4) generating a plurality of second microdroplets from the polymer solution, wherein the first and second microdroplets are generated by at least one atomizing device; and (5) contacting the first and second microdroplets to form a plurality of pre-encapsulant particles. The pre-encapsulant particles have a core domain containing the aforementioned composition and an outer layer containing the polymer, such that solvent exchange occurs between the core domain and the outer layer thereby forming a polymer shell around the encapsulated composition.
Atomizers for use in conducting a production method of the present invention include those that employ centrifugal, pressure, kinetic, and sonic energy. Preferred atomizers are ultrasonic ones, as illustrated by examples hereinafter. Substances that can be encapsulated include any water-soluble compound, but particularly interesting are bioactive substances, such as low molecular weight drugs, proteins, oligonucleotides, gene, and polysaccharides. A particularly advantageous property of microcapsules produced according to the present invention is that they can exhibit controlled release properties.
A number of atomizer configurations for use in generating microcapsules according to the present invention are contemplated. A particularly surprising and useful configuration is one in which a coaxial ultrasonic atomizer is submerged into an aqueous collection bath. In this configuration, immediate solvent exchange can occur between the microparticles and the bath in addition to the solvent exchange that can occur between core and shell domains.
Pre-encapsulant particles formed by a method of the present invention can be hardened, i.e., the polymer-containing shell can be solidified, through solvent exchange. Such hardening can be due to solvent exchange between the aqueous core and polymer-containing shell of the particles and/or between the shell and surrounding bath. Alternatively, solvent removal from the polymer-containing shell can be effected by spray drying or freeze drying techniques.
In another aspect of the present invention, microbubbles can be produced using an atomizer. In this aspect, hydrophobic drugs can be loaded alongside polymer in a single phase. Microcapsules containing hydrophobic compounds within a non-aqueous core can also be made.